A conventional ball joint has been prevailingly used as a joint in a suspension system and/or a steering system of an automobile. As shown in FIG. 10, the conventional ball joint includes: a ball stud 102 having a spherical ball part 104 and a shank part 103 extending from the spherical ball part 104; a bearing 106 made of a synthetic resin, cupping the spherical ball part 104 of the ball stud 102 in such a manner that the spherical ball part 104 is able to turn and rock therein, and having an open part 107 in one end; a substantially cup-shaped housing 111 holding the bearing 106 therein and having an open part 114 through which the shank part 103 of the ball stud 102 projects; an annular retaining ring 122 fitted in an opening defined by the open part 114 of the housing 111; and an elastic rubber boot 116 having a small open part 118 fitted on the shank part 103 of the ball stud 102, and a large open part 117 fitted on the retaining ring 122.
The inner surface of the bearing 106 is coated with grease in order to ensure smooth turning and rocking of the spherical ball part 104 of the ball stud 102 in the bearing 106. Generally, the inside diameter of the bearing 106 is substantially equal to the outside diameter of the spherical ball part 104. Therefore, the grease on the inner surface of the bearing 106 may be forced to flow outside through the open part 107 by the spherical ball part 104 when the spherical ball part 104 is pressed into the bearing 106. Consequently, a quantity of the grease in the bearing 106 may be reduced, that is, the bearing 106 may become unable to hold a necessary and sufficient quantity of the grease. In the case, the spherical ball part 104 may be unable to smoothly turn and rock.
In addition, torque necessary for the ball stud 102 to turn and rock is affected by a load exerted by the bearing 106 on the spherical ball part 104. In the ball joint 101 shown in FIG. 10, the entire inner surface of the bearing 106 is in contact with the spherical ball part 104 of the ball stud 102, so that the load is exerted by the entire inner surface of the bearing 106 on the spherical ball part 104. Therefore, higher torque tends to be necessary for the ball stud 102 to turn and rock.
FIG. 11 shows a ball joint 201 proposed to solve the foregoing problem. A bearing 206 included in the ball joint 201 is provided with a circumferential groove 209 in an inner surface thereof. The circumferential groove 209 serves as a grease holding means.
However, when the circumferential groove 209 is formed beforehand in the inner surface of the bearing 206, flow of the grease is obstructed by contacting portions between the spherical ball part 204 and the inner surface of the bearing 206, as shown in FIG. 12. Consequently, the grease may remain only in the circumferential groove 209, so that the grease may be unable to satisfactorily lubricate a desired wide range.